Processing for the high strength alpha beta titanium alloys

ABSTRACT

A PROCESS FOR IMPROVING THE TOUGHNESS AND PROPERTY UNIFORMITY OF THE HIGH STRENGTH ALPHA-BETA TITANIUM FORGED ALLOYS BY A SPECIFIC HEAT TREATMENT SCHEDULE WHICH INCLUDES A SHORT ISOTHERMAL TRANSFORMATION HEAT TREATMENT STEP LOW IN THE ALPHA-BETA REGION SUBSEQUENT TO SOLUTIONING TREATMENT HIGH IN THE ALPHA-BETA REGION.

United States Patent 3,748,194 PROCESSING F OR THE IHGH STRENGTH ALPHA-BETA TITANIUM ALLOYS Duane L. Ruckle, Enfield, Robert A. Sprague, Kensington, and Michael P. Smith, Glastonbury, Conn., asiignors to United Aircraft Corporation, East Hartford,

onn. No Drawing. Filed Oct. 6, 1971, Ser. No. 187,037

Int. Cl. C22c 15/00; C22f 1/18 US. Cl. 14812.7 4 Claims ABSTRACT OF THE DISCLOSURE A process for improving the toughness and property uniformity of the high strength alpha-beta titanium forged alloys by a specific heat treatment schedule which includes a short isothermal transformation heat treatment step low in the alpha-beta region subsequent to solutioning treatment high in the alpha-beta region.

BACKGROUND OF THE INVENTION The present invention relates to the processing of the high strength alpha-beta titanium alloys to improve the level and uniformity of their mechanical properties.

The high strength alpha-beta titanium alloys, such as Ti 6-2-4-6 (6 percent aluminum, 2 percent tin, 4 percent zirconium, 6 percent molybdenum, balance titanium) and the Ti 6-6-2 alloy (6 percent aluminum, 6 percent vanadium, 2 percent tin, balance titanium), when processed by the conventional heat treatments exhibit a broad scatter in toughness, strength and fatigue resistance. The most common heat treatment for the Ti 6-2-4-6 alloy comprises: solutioning for about 1 hour at 1660 F.; air cooling; precipitation heat treatment at 1100 F. for 4-8 hours; and cooling in air.

The scatter in properties resultant from such heat treatment is directly related to microstructural differences within a given component and between components. In the standard processes, whether incorporating air cooling, oil or water quench after solutioning, the cooling rates between various sections of a given article cannot be well controlled due to difierences in forging section size. Thus, for a given cooling technique from the solution temperature the thin sections tend to exhibit higher tensile and yield strengths and lower fracture toughness than thicker sections of the same forging. These variations in properties can be correlated with variations in alloy microstructure. In particular, there is the development of large amounts of very small secondary alpha plates in areas subject to rapid cooling and large, coarse alpha plates in the thicker sections which have cooled more slowly.

' These property variations between different sections of a given article and between articles of different configuration and size are generally disadvantageous for the sensitive applications such as gas turbine engine hardware, where such alloys find their greatest utility.

SUMMARY OF THE INVENTION It is a principal object of the present invention to improve the toughness and property uniformity of the high strength, age-hardenable, alpha-beta titanium alloys.

In the furtherance of this objective there is contemplated herein a heat treatment process for such alloys which comprises: a solution heat treatment optionally conducted concurrently with, but usually subsequent to, for ing to adjust the quantity and morphology of the alpha phase in the microstructure; a direct transfer to an isothermal media, held at a high temperature but lower than the solutioning temperature, for a time period sufficient to achieve the desired transformation products and 3,748,194 Patented July 24, 1973 phase chemistries; and a subsequent aging, at a temperature suitable for the alloy, for strenghening purposes.

In a particular preferred embodiment of the invention, the Ti 6-2-4-6 alloy is processed by: solution heat treatment, typically 1690 F. for 2 hours; direct transfer from solution heat treatment to a molten salt bath in the general range of about 1400-1600 F., typically 1525 F. with a hold for about fifteen minutes; and subsequent aging at 950-l150 F., typically 1100 F. for 8 hours; and air cooling.

DESCRIPTION OF THE PREFERRED EMBODIMENT The high strength titanium alloys are desired for current lightweight, high performance turbine powerplants where a high specific modulus and creep strength are fundamental design criteria. Of intense interest are the age-hardenable, alpha-beta titanium alloys.

The Ti 6-6-2 alloy has been extensively investigated but, although high yield strengths are attainable with this alloy, it suffers somewhat from limited hardenability and low creep strength.

A recently developed high strength, age-hardenable, alpha-beta titanium alloy, which has its basis in the Ti 6-2-4-2 alloy, is the Ti 6-2-4-6 alloy earlier mentioned. The Ti 6-2-4-6 alloy has demonstrated high yield strengths and, in addition, has a greater hardening potential and higher creep strength than the Ti 6-6-2 alloy. Accordingly, it is the preferred alloy of current interest.

This alloy was purchased from the producer as eight inch round billet with an actual composition (K-2407) of 6.2 percent aluminum, 2.1 percent tin, 4.2 percent zirconium, 6.1 percent molybdenum, 0.06 percent iron, 0.12 percent oxygen, 0.008 percent nitrogen, 0.007 percent hydrogen, balance titanium. The alpha+beta to beta transus for this heat was determined to be 1735J -10 F. Several billet sections were cross-worked by multiple upset and redraw operations at 1625 F. to reduce the elongated alpha particle content and create a more homogeneous billet structure. Open die pancake forgings 1.75 inches thick and 18 inches in diameter were produced at a number of forging temperatures from 1625 F. to 1800 F. All pancakes were then cut into two or more sections and the effects of solution heat treatment at temperatures from 1525 F. to 1730 F. were investigated. Aging between 950" F. to 1100 F. usually completed each processing treatment.

The effects of cooling rate from forging temperature were studied. In addition, the quench rate from the solution treatment temperature was investigated in substantial detail by cooling various segments in different ways including air cool, oil quench and water quench. Mechanical property measurements and micrographs were obtained both near the surface and at the center of each segment, since these locations experienced different thermal histories.

It was readily apparent that, with the exception of a few data points, it was not possible to obtain the desired properties with the conventional processing treatments which had been employed. Those data points meeting the requirements were from pancakes which had been beta forged and water quenched from the forging press, a practice considered impractical for thicker sections.

The trend for fracture toughness of these alloys to increase somewhat with decreasing percentages of primary alpha (equiaxed globular alpha) is shown when the amount of primary alpha is less than about 35 percent. This result is not totally unexpected since the ultimate condition of having no primary alpha is the tougher completely transformed beta structure which results from beta processing. However, the lack of a definite trend for all strength levels at higher percentages of primary alpha indicates that this variable alone is not sufiicient to account for the effect of microstructure on the mechanical properties.

' As mentioned above, the beta processed microstructures can be readily distinguished from the alpha-beta processed structures by their lack of primary alpha. In the alpha-beta processed material the microstructures which yield the highest fracture toughness at yield strength levels between 170 and 180 k.s.i. may be defined as containing about 10 percent globular alpha (primary alpha) with a matrix of relatively coarse acicular alpha (secondary alpha) and aged beta. An acceptable level of tensile ductility (20% RA) is also obtained with this microstructure.

The basic microstructure is, of course, tailored to some extent depending upon the particular goal properties desired. For a gas turbine engine compressor disk, for example, in the strength/toughness trade off as part of the alloy property optimization process, more coarse alpha is built into the alloy providing increased toughness somewhat at the expense of strength. For a compressor blade application, however, the strength/toughness trade off would typically be reversed, providing increased strength even if the achievement thereof were provided somewhat at the expense of toughness.

The fracture toughness of a given specimen is a function of the yield strength. When the applied stress is set equal to a constant fraction of the yield strength of the alloy, the critical crack size for unstable fracture in plane strain is known to be proportional to (K /c where K is the critical plane strain stress intensity factor and 4. transformation temperature rather than by highly variable continuous cooling rates and section size sensitivity is therefore reduced. Alpha and beta phase transition chemistries resulting from elemental partitioning are also controlled bythe isothermal treatment so that properties may be optimized.

The basic forging microstructure comprises substantially equiaxed primary alpha in a transformed beta matrix resultant from forging at a temperature up to 1700 F., typically 1629-1650 F., in'the case of the Ti 62-46 alloy. Forgings with coarser elongated alpha platelets which are less fragmented or exhibit less random orientation are considered rejectable, as is a lack of primary alpha.

Solution heat treatments are conducted generally at temperatures up to about 1700 F., typically at about 1690 F. for l 2 hours, the holding time being dependent on section size but being sufficient in any event to provide relatively uniform heating. The solution temperature, however, must be high enough to control the size of the beta phase since the beta subsequently controls the size of the alpha platelets. At solution temperatures too high above the beta transus subsequent ductility is very poor. Solutioning is therefore conducted basically 6} is the yield strength. It is possible, however, to modify microstructural features to increase fracture toughness at a given strength level by creating random preferred crack growth paths in the structure. These preferred crack growth paths are along alpha plate interfaces and control of the size and orientation of alpha plates is necessary to achieve fracture toughness at high strength levels in alpha-beta titanium alloys. At a typical yield strength goal of 170 k.s.i. the K goal was k.s.i. Vin.

The K goal was achieved by solution heat treatment to adjust the quantity and morphology of the equiaxed alpha phase with a direct transfer to an isothermal environment providing controlled isothermal transformation of the beta phase to coarse alpha plates. Subsequent conventional aging is then performed.

The isothermal transformation sequence allows exce lent control of the matrix phase morphology to achieve optimum microstructures and properties. The amount of coarse secondary alpha is determined by the isothermal below the beta transus temperature, typically about 10- 50 F. therebelow, but within about F. thereof.

Transfer from solutioning is direct to the isothermal media which can be hot salt or any reasonably good heat conductor suitable for the general purpose. Chloride salts and the cyanides are suitable. Basically any salt is usable where the resultant surface corrosion can be accommodated.

The microstructure and consequently, alloy properties are both a function of the temperature of the isothermal media and the holding time therein. The isothermal transformation treatment allows sutficient control to permit a complete transformation to coarse alpha or a mixed microstructure, both coarse and line alpha plates, if the shorter holding times or higher transformation temperatures are utilized. The higher strength but with some compromise in toughness, results from the shorter holding times or higher transformation temperatures.

Thus, isothermal transformation is conducted Within the general temperature range of the alpha transus to about 200 F. above, in the range, therefore, of about 14001600 IF., preferably 1500l575 F.

From the isothermal media cooling may be accomplished by water quench or air cooling, the air cooling procedure involving some strength compromise.

Aging is conventional, and usually conducted in the range of about 950-1100 F.

ROOM TEMPERATURE TENSILE AND FBACTURE TOUGHNESS PROPERTY COMPARISON OF Tl-6Al-2Sn-4Zr-6Mo ALLOY FOR CONVENTIONAL vs. ISOTHERMAL HEAT TREATMENTS Conventional heat treatment 0. 2% Km UTS, Ys, El, Ra, Heat treatment k.s.i. k.s.i. percent percent /i n 1,525 F. (1 hr.) AC+1,100 F. (8 hrs.) AC {8 3 123:8 13:2 33 3%? k.s.l. ultimate strength level.

181.0 171. 4 9. 5 21. 7 2s. 7 1,600 F. (1 hr.) AC+1,100 F. (8 hrs.) AC 184 0 172 0 10 5 7 4 k.s.1. ultimate strength level. 1,650 F. (4 hrs.) AC+1,100 F. (8 hrs.) AC iggg E5 2 gg 2;; g h.s;1. hmmsts strength level. 1.685 F. (1 hr.) A0+1,1oo F. (s 1118-) A0 i332 {$22 8 3&8 3Z3 Do. is: s e; eniuse -1 ram-mass"- as as .2-2 2 s; m 0 n. s. 0. 1,690 F. 2 hrs.) so, i,525 ()6 hr.) AG+1,100 (8 hrs.) AC..- 167. 0 12.1 28.1 60. 5 i ultimate strength level- 1,0s0 2 hrs.) FT, 1,525 14 hr.) 11c+1,10o 3 hrs.) AC 184. 0 175. 2 12. 20. 0 37. 6 180 k.s.l. ultimate strength level. 1,690 F. (2 hrs.) so, 1,425 F. (5 mm.) AG+1,100 F. 8 hrs.) no 177.8 152.7 11. 5 21.1 23:? Do. 1,690 F. (2 hrs.) so, 1,450 F. 15 min.) AO+1,100 F. 8 is.) AC".-- 177.0 159.0 10. s 23.0 2?; E; Do. men F. (2 hrs.) so, 1,525 5 min.) AC+1,100 F. (8 hrs.) AC 190.8 173. s 12. 6 36. o it: }1s0 h.s.1. ultimate strength level, 1,090 F. 1 hr.) SQ,1,600 F. 10 min.) ho+1,100 (8 hrs.) AC.-- 203. a 185. 2 s. 7 15. 5 a7. 8 200 k.s.l. ultimate strength level NorE.AC=Alr cool to room temperature; FT=Furnaee Transfer to indicated temperature; OQ=Qilquench to room temperature; SQ=Salt quench to indicated temperature.

In summary, for the Ti 6-2-4-6 alloy, following solution heat treatment, controlled isothermal transformation between 1400-1600 F. for times between 30 seconds and 2 hours is satisfactory, as is aging between 950-1100 F. The preferred cycle comprises: solution heat treatment at 1690 F. for about 2 hours; direct transfer to a molten salt bath at 1525 F. hold for fifteen minutes and cool to room temperature; age 1100 F. for 8 hours; and air cool. This provides the optimum strength and toughness in this alloy resulting from the presence of both fine and coarse secondary alpha plates in the microstructure.

The preceding table illustrates the respective room temperature tensile and fracture toughness properties of the Ti 6-2-4-6 alloy as processed by conventional means and by the processing of the present invention.

In each case, step 1 involves a solution heat treatment high in the two-phase region to develop the small amount (5-30%, preferably about 10%) of primary alpha desired. If forging, rolling or extrusion is, in fact, done at a sufficiently high temperature, then it is indeed a solution treatment. Aging is conventional in all cases. It is the control of transformation that provides the dramatic improvement in properties. The overall process is controlled to form the proper amount of the acicular alpha phase while not producing additional equiaxed alpha, and having sufficient retained beta so that the desired strength level can be obtained upon subsequent aging.

The invention in its broader aspects is not limited to the specific details described but departures may be made therefrom within the scope of the accompanying claims without departing from the principles of the invention and without sacrificing its chief advantages.

What is claimed is:

1. The method of processing high strength alpha-beta titanium alloy forgings of the type typified by the alloy consisting of, by weight, 6 percent aluminum, 2 percent tin, 4 percent zirconium, 6 percent molybdenum, balance titanium, which comprises:

solution heat treating the alloy within the alpha/beta region at a temperature of 16001700 F. for a minimum of about 1 hour to develop 5 to 30% of the primary alpha phase;

isothermally transforming the alloy in the alpha/beta region at a temperature of 14001600 F. to develop the desired amount of the secondary alpha phase;

cooling the alloy; and

subsequently aging the alloy at a temperature of about 2. The method according to claim 1 wherein:

the isothermal transformation is conducted at a tem perature of about 1525 F. for about minutes.

3. The method of processing the alloy consisting essentially of, by weight, 6 percent aluminum, 2 percent tin, 4 percent zirconium, 6 percent molybdenum, balance titanium, which comprises:

forging the alloy within the alpha/beta region to provide a substantially equiaxed alpha phase in the microstructure;

solution heat treating the alloy high within the alpha/ beta region and generally within the range of 1600- 1700 F. to develop 5 to 30% of an equiaxed alpha phase;

isothermally transforming the alloy low within the alpha/beta region and generally Within the range of 1400-1600 F. to provide the desired amount of an acicular alpha phase;

cooling the alloy; and

subsequently aging the alloy in a strengthening heat treatment generally within the range of about 950 1100" F.

4. The method of processing the alloy consisting essentially of, by weight, 6 percent aluminum, 2 percent tin, 4 percent zirconium, 6 percent molybdenum, balance titanium, which comprises:

forging the alloy at a temperature of about 1625"- solution heat treating the alloy at a temperature of about 1690 F. for about 1-2 hours;

isothermally transforming the alloy in an isothermal fluid at a temperature of about 1525 F. for about 15 minutes;

cooling the alloy; and

subsequently aging the alloy at a temperature of about 950-1100 F. for a minimum of about 8 hours.

References Cited UNITED STATES PATENTS 2,918,367 12/1959 Crossley et a1. -1755 2,968,586 1/1961 Vordahl 75175.5 X 2,754,203 7/1956 Vordahl 75-1755 2,769,707 11/1956 Vordahl 75-1755 OTHER REFERENCES Metallurgical and Mechanical Properties of Titanium Alloy T1-6Al-2Sn-4Zr-2Mo, Sheet Bar and Forgings, September 1966, 18 pages.

CHARLES N. LOVELL, Primary Examiner US. Cl. X.R. 148-325, 133 

